using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

using System.Net.Http;

using System.Runtime.Serialization;

​

public class Example

{

    class Program

    {

        class T2

        {

            public string T { get; set; }

            public Dictionary<string, object> U { get; set; } = new Dictionary<string, object>();

        }

​

        public static void Main()

        {

            string o = "{\"T\":\"\"}";

            var r = JsonConvert.DeserializeObject<T2>(o);

            Console.WriteLine(JsonConvert.SerializeObject(r));

            string t = "{\"T\":\"\", \"U\": null }";

            var u = JsonConvert.DeserializeObject<T2>(t);

            Console.WriteLine(JsonConvert.SerializeObject(u));

        }

    }

}

​